Laser systems for treating the eye involve directing an energetic laser beam upon the cornea to remove corneal tissue by the process known as ablation. Excimer lasers are typically employed. Maintaining the relationship of each instant of laser energy deposition relative to previous depositions is important to produce the desired conformational change across the treated corneal area. In order to maintain registration, tracking systems have been employed based on the pupil and iris of the eye.
One established excimer laser system has an iris tracking system employing small, low energy tracking beams, to track the edge of a fixed and dilated pupil using a dedicated edge detection system. Four infrared beams scan back and forth in radial fashion at different quadrants of the round pupil, each detecting a signal difference as the beam crosses the edge of the dark pupil onto the iris. The pupil is typically required to be dilated and not moving. A clinical strategy for employing this technique first captures a video digital image of the non-dilated eye when the patient is fixated on a spot, such as on a blinking red light. The limbus and/or other anatomical landmarks are identified by a human operator, and a computer-generated image of a ring is imposed on the limbus on the computer screen. Using that ring, the laser surgeon then locates a yellow-cross-hair in the center of the pupil as a reference for the treatment. The patient then is taken from the treatment station, administered a drug to dilate the pupil, and when stable dilation is achieved, the patient returns for laser treatment. With the tracking system activated, the computer-captured-ring from the pre-dilation state is imposed on the image of the limbus of the dilated eye, when fixated as before. This locates the yellow cross-hair, representing the pupil center in the same position as before dilation. Though the pupil is dilated to larger, fixed size, the laser surgeon, knowing the center of the undilated pupil, can conduct the procedure with that reference.
A system along that line is disclosed in U.S. Pat. No. 5,740,803, hereby incorporated by reference.
In another established laser treatment system, the front of the eye is flooded with infrared (invisible) light to detect the transition between the iris, which significantly reflects the infrared light, and the pupil, which is relatively absorptive of the infrared light. An infrared sensitive photo detector measures the reflected infrared light. On the assumption that the pupil is round, the computer performs edge detection to determine best fit and mathematically solves the equation to determine the center of the round circle determined from the detected data. This system allows the pupil to be smaller and to vary somewhat in size during treatment, employing the assumption that the pupil expands and contracts concentrically. As long as the eye is concentric and moves within a relatively modest range, e.g. between 2 and 4 millimeters, the mathematical solution for the center of the circle remains essentially at the same spot and the treatment remained centered.
In further proposed systems, pattern recognition techniques have been proposed for detecting the iris, by which the center and orientation are determined, from which the desired corrections are referenced.
Besides such prior systems that conduct laser treatment based on an image of the iris of the eye in the X, Y plane, other proposals have concerned tracking other gross features such as the incision line of a turned corneal flap, or of using techniques based on optical features internal of the eye.